Chapter 9: Virtual Memory
Operating System Concepts – 9th Edition
Silberschatz, Galvin and Gagne ©2013
Background
 Virtual memory – separation of user logical memory from
physical memory

Only part of the program needs to be in memory for execution

Logical address space can therefore be much larger than
physical address space

Allows address spaces to be shared by several processes

Allows for more efficient process creation

More programs running concurrently

Less I/O needed to load or swap processes

Virtual memory makes the task of programming much easier

the programmer no longer needs to worry about the
amount of physical memory available;

she can concentrate instead on the problem to be
programmed.
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Background (Cont.)
 Virtual address space – logical view of how process is stored in memory

Usually start at address 0, contiguous addresses until end of space

Meanwhile, physical memory organized in page frames

MMU must map logical to physical
 Virtual memory can be implemented via:

Demand paging

Demand segmentation
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Virtual Memory That is Larger Than Physical Memory
backing
store
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Page Table When Some Pages Are Not in Main Memory
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Page Fault
 If the process tries to access a page that was not brought into memory,
 Or tries to access any “invalid” page:

That will trap to OS, causing a page fault

Such as when the first reference to a page is made
1. Operating system looks at another table to decide:
Invalid reference  abort the process
 Just not in memory => continue
2. Find free frame

3. Swap page into frame via scheduled disk operation
4. Reset tables to indicate page now is in memory

Set validation bit = v
5. Restart the instruction that caused the page fault
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Steps in Handling a Page Fault
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What happens if there is no free frame?
 Page replacement


Find some page in memory that is not really in use and swap it.

Need page replacement algorithm

Performance Issue - need an algorithm which will result in minimum
number of page faults.
Same page may be brought into memory many times.
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Basic Page Replacement
1.
Find the location of the desired page on disk
2. Find a free frame:
- if (a free frame exist) then use it
- else
3.
use a page replacement algorithm to select a victim frame

write victim frame to disk, if dirty
Bring the desired page into the (newly) free frame;

4.

update the page and frame tables accordingly
Continue the process by restarting the instruction that caused the trap
 Note: now potentially 2 page transfers for page fault

Increasing EAT
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Page Replacement
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Page Replacement Strategies
 The Principle of Optimality







Replace the page that will not be used again the farthest time into the
future.
Random Page Replacement
 Choose a page randomly
FIFO - First in First Out
 Replace the page that has been in memory the longest.
LRU - Least Recently Used
 Replace the page that has not been used for the longest time.
LFU - Least Frequently Used
 Replace the page that is used least often.
NUR - Not Used Recently
 An approximation to LRU
Working Set
 Keep in memory those pages that the process is actively using
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First-In-First-Out (FIFO) Algorithm
 Reference string: 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1
 3 frames (3 pages can be in memory at a time per process)
15 page faults
 How to track ages of pages?

Just use a FIFO queue
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FIFO Illustrating Belady’s Anomaly
 Reference String: 1,2,3,4,1,2,5,1,2,3,4,5
 Assume x frames (x pages can be in memory at a time per process)
3 frames
Frame 1
Frame 2
Frame 3
1
2
3
4
1
2
5
3
4
4 frames
Frame 1
Frame 2
Frame 3
Frame 4
1
2
3
4
5
1
2
3
4
5
9 Page faults
10 Page faults
FIFO Replacement - Belady’s Anomaly -- more frames does not mean less page faults!
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Optimal Algorithm
 Replace page that will not be used for longest period of time

9 page faults is optimal for the example
 How do you know this?

Can’t read the future
 Used for measuring how well your algorithm performs
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Least Recently Used (LRU) Algorithm
 Use past knowledge rather than future
 Replace page that has not been used in the most amount of time
 Associate time of last use with each page
 12 faults – better than FIFO but worse than OPT
 Generally good algorithm and frequently used
 But how to implement?
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Implementation of LRU Algorithm
 Counter implementation

Every page entry has a counter; every time page is referenced
through this entry, copy the clock into the counter

When a page needs to be changed, look at the counters to find
smallest value

Search through table needed
 Stack implementation

Keep a stack of page numbers in a double link form:

Page referenced:


move it to the top

requires 6 pointers to be changed
No search for replacement

But each update more expensive
 LRU and OPT don’t have Belady’s Anomaly
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LRU Approximation Algorithms (contd.)
 Second-chance algorithm (=Clock algorithm)

Based of FIFO replacement algorithm


Also uses the hardware-provided reference bit
If page to be replaced has

reference_bit = 0 => replace it

reference_bit = 1 =>
–
set reference bit 0, leave page in memory
–
replace next page, subject to same rules
 Idea:

If reference_bit = 1, we give the page a 2nd chance and move on to
select the next FIFO page.

When a page gets a second chance, its reference bit is cleared, and its
arrival time is reset to the current time.

Thus, a page that is given a 2nd chance will not be replaced until all
other pages have been replaced (or given second chances).

In addition, if a page is used often enough to keep its reference bit set, it
will never be replaced.
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Second-Chance (clock) Page-Replacement Algorithm

A circular queue implementation.

Uses a pointer

that is, a hand on the clock

indicates the page to be replaced next.

When a frame is needed, the pointer
advances until it finds a page with a 0
reference bit.

As it advances, it clears the reference bits.

Once a victim page is found,


the page is replaced, and

the new page is inserted in the circular
queue in that position.
BSD Unix used this
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